1. Field of the Invention
The invention relates to hybrid electric vehicle powertrains having an internal combustion engine and an electric motor that cooperate to provide a first power delivery mode with electromechanical power flow paths and a second power delivery mode in which the motor provides driving power independently of the engine.
2. Background Art
One class of hybrid electric automotive vehicles includes parallel-series hybrid electric vehicles. Such vehicles include a powertrain for delivering power to traction wheels from two power sources through gearing. In one powertrain configuration of this type, a combination of an internal combustion engine and a subsystem consisting of an electric motor and a generator uses a planetary gearset to define in part separate torque delivery paths to the traction wheels. The subsystem comprising the generator and the motor includes a battery, which acts as an energy storage medium.
When the engine, the motor and the generator are functioning in a first driving mode, the engine propels the vehicle in a forward direction using reaction torque from the generator. The planetary gearset in this configuration makes it possible for the engine speed to be effectively decoupled from the vehicle speed through a generator speed control. As a result, engine output power is divided between a mechanical power flow path and an electrical power flow path. The mechanical power flow path extends from the engine to a planetary carrier, to a planetary ring gear, to transmission countershaft gears and, finally, to traction wheels. The electrical power flow path extends from the engine to the planetary carrier, to a planetary sun gear, and to a generator, the generator being electrically coupled to the motor. The motor drives the transmission countershaft gears and the traction wheels. Because of the decoupling of the engine speed from the vehicle speed and the electrical and mechanical power flow paths, such parallel-series hybrid electric vehicle powertrains emulate the characteristics of a continuously variable transmission during a first driving mode.
In a second driving mode, the engine is inactive and the motor, generator and battery subsystem acts as a power source. The electric motor then draws power from the battery and provides propulsion independently of the engine at the traction wheels in both forward and reverse directions.
The electric motor can provide braking torque to capture vehicle kinetic energy during braking, which otherwise would be lost in the form of heat. This charges the battery as the motor acts as a generator. Furthermore, the generator, using battery power, can drive against a one-way clutch on the engine power output shaft to propel the vehicle in a forward direction. In this driving mode, the generator acts as a motor.
When the generator acts as a generator, the vehicle can be propelled in a forward direction to meet a driver's demand for power and to achieve improved acceleration performance.
In a powertrain with conventional continuously variable transmission characteristics it possible to achieve better fuel economy and exhaust gas emission quality by operating the engine at or near the most efficient operating region of its engine speed and torque relationship. It is possible with a parallel-series hybrid electric configuration, however, to potentially improve fuel economy and emission quality even further compared to conventional continuously variable transmission vehicles. This is because the engine size can be reduced while retaining similar vehicle performance due to the presence of dual power sources. Also, overall engine operation can be better optimized since the engine can be stopped if the engine operating conditions are not favorable for high fuel economy operation or if the engine is not in a high emission quality control region. Furthermore, kinetic energy during braking can be captured and stored in the battery through regenerative braking.
Improved fuel economy and emissions control using a parallel-series hybrid electric powertrain configuration is achieved, however, at the expense of system complexity because of the dual power sources. Further, weight and cost may be design penalties. To offset these considerations, the dual power sources can be integrated to work together seamlessly to achieve the goal of better fuel economy and emissions control.
One of the measures that can be taken to coordinate control of the two power sources to achieve better fuel economy and exhaust gas emission quality in a hybrid electric vehicle is to shut off the engine when the engine cannot be operated in a desired efficient operating region; for example, when the vehicle is stopped at traffic lights. This is unlike the function of a conventional engine powertrain where the engine must be started during the first startup of the vehicle and shut off only by turning the ignition key to its off position. In contrast, the engine in a parallel-series hybrid electric vehicle powertrain can be started and stopped repeatedly during normal city driving.
These engine start and stop events can be unexpected to the driver under certain circumstances. They are required, therefore, to be smooth or imperceptible to the driver.